Abstract

This is the first report of three-dimensional, immature, dipteran impressions in tufa from the Green River Formation. These unique fossils occur in tufa that encrusted submerged logs at the margin of Eocene Fossil Lake, southwest Wyoming. The immature dipteran impressions record paleocurrent information because they are usually aligned parallel with each other, often with the wider end (pupal head and thorax, or larval abdomen) oriented toward a uniform direction (upstream/current). The alignment of the immature dipterans with the current implies that they were rheophilic (preferring to live in running water). In addition, the long axis of the fossils is aligned with the long axis of the underlying wood. Several successive generations of immature dipterans were preserved in the tufa. The fossils indicate that strong paleocurrents were active along the eastern shoreline of Fossil Lake (or that strong stream or spring currents entered the lake at this location). Rapid calcite deposition was required to encrust these soft-bodied, motile dipteran pupae or larvae. We favor the hypothesis that the tufa formed at the interface between a calcium-rich freshwater spring or stream and the saline-alkaline Fossil Lake.